1. Field of the Invention
The present invention relates a rolling die for a ball screw which is used to manufacture the screw shaft of a ball screw.
2. Description of the Related Art
Generally, the screw shaft of a ball screw is formed of a rod-shaped screw shaft raw material made of metal and includes a helical ball groove formed therein. As a method for manufacturing such screw shaft, in JP-A-9-133195, there is disclosed a method in which such a rolling die 10 as shown in FIG. 7. That is, a rolling die 10, which comprises a cylindrical portion 12 including a helical protrusion formed on the outer peripheral surface thereof and further comprises a conical lead-in portion 13 and a conical run-off portion 14 respectively formed in the two end portions of the cylindrical portion 14, is pressed against such a raw material of a screw shaft w as shown in FIG. 8. In this pressed state, one of the rolling die 10 and screw shaft raw material w is rotated with respect to the other to thereby roll and work a ball groove g in the raw material w of the screw shaft. The method disclosed in the cited publication is a so called through feed rolling-type rolling method which uses the walking phenomenon (moving of the product in an axial direction thereof in the rolling process) of the raw material of the screw shaft. When compared with a method for manufacturing a screw shaft by turning or grinding the raw material of the screw shaft to form a ball groove therein, the through feed rolling-type rolling method is excellent in mass production and is able to manufacture a precision ball screw at a relatively low cost.
However, when manufacturing a screw shaft including a ball groove having a pitch larger than the ball diameter using the thread rolling die shown in FIG. 7, that is, as shown in FIG. 9, a screw shaft bs with the pitch p of the ball groove g thereof being 1.7 times or more than the diameter dB of the ball b, there is a possibility that the crushing amount of the raw material per crush by the thread rolling die can be large. This raises a possibility that the circularity deviation and shape transfer property of a ball groove to be rolled and formed in the raw material of the screw shaft can be lowered, thereby being unable to satisfy the required specifications. In order to satisfy the required specifications, in case where the length of the lead-in portion of the rolling die is increased to thereby reduce the raw material crushing amount per crush, the number of working operations increases, thereby causing the raw material of the screw shaft to harden through the working operations, so that exfoliations may occur on the working surface of the raw material of the screw shaft.
Also, in case where a ball groove is rolled and worked in a raw material of a screw shaft using the rolling die shown in FIG. 7, there occurs excessive stress concentration in the boundary portion between the cylindrical portion and lead-in portion of the rolling die, which can shorten the durable life of the rolling die.